Fix: realFetch.call is not a function

[merutin]

#194 Fixed an error when building with Webpack.

Reproduction is available in the following repositories. https://github.com/merutin/isomorphic-fetch-repro

Please let me know if there are any necessary steps.

[karna41317]

@matthew-andrews any update on this ?

[mletterle]

This would be a real nice to have...